Each year, the northern elephant seals embark on a seven-month, 10,000-mile (10,000-kilometer) journey across the North Pacific in search of fish and squid to eat. They begin the journey after sitting on the beach for a few months – replacing their fur and losing fat – and gradually picking it up over the course of the feeding journey. On these excursions, elephant seals do not just swim on the surface – they dive incessantly, day and night.
© Dan Costa
Elephant seals take feeding trips for seven months during which they balance danger, hunger and exhaustion.
To rest, they swim hundreds of feet below the ocean surface, then slowly roll on their backs and float back and forth like falling leaves. These dives last about 25 minutes and are called drift dives. The rest period of drift diving is the most dangerous time for an elephant seal: Waking up in the jaws of a white shark or orca is not a good way to start the day.
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We are two biologists studying diving behavior and sleeping with mammals. Specifically, we are fascinated by the decisions elephant seals make as they roam the open ocean and navigate extreme changes in their environment and their own bodies. The open ocean is a dangerous place, and animals must constantly weigh the risks of robbery, hunger and exhaustion. The choice of when to rest and when to feed has serious consequences.
Elephant seals have two options: rest during the safety of the dark night and feed during the day when it is more difficult to obtain food, or rest during the day when there is a much greater chance of being eaten by a predator and during feed the night when fish and squid are more available.
Therefore, we asked ourselves: do lean, hungry seals take more risks than plump, healthy seals? Our latest study, recently published in the journal Science Advances, shows that elephant seals have refined their risk-taking behaviors throughout the food journey.
Risk versus reward
One simple question is fundamental to ecologists’ understanding of the natural world: do hungry animals take more risks in finding food? This should be true in theory, because wild animals constantly weigh the risks of famine and predators. For most species, it is almost impossible to measure continuous changes in health. As a result, many theories about risk and reward in the animal kingdom have existed for decades, but have yet to be tested.
The ocean is a fascinating place to study risks and rewards, because light levels determine life and death in three dimensions: The surface of the ocean is clear and predators can hunt much more easily; but the light fades quickly as you dive deeper into the ocean. For elephant seals, light levels are directly related to risks, because their main predators live in shallow waters and use light to hunt. For elephant seals, rest at night is safer if predators cannot find it.
© Illustrations by Danielle Dube, Infographic by Jessica Kendall-Bar
Sleeping seals run the greatest risk during the day and in shallow water, while the risk is lowest during the night and in deep water.
Light levels are also directly related to reward, as most elephant prey – fish and squid – migrate up and down the water column every day. During the day, when light levels are high, fish and squid stay in the depths to avoid predators. At night, however, when the light levels are low, fish and squid swim closer to the surface to feed plankton. For seals, feeding at night is more effective when prey has risen from the depths to find their own food.
This means that the best time to eat is also the safest time to rest, and seals should choose one behavior over the other. Do they prioritize resting safely or feeding effectively? And does it change over time as they get fat?
© Dan Costa
An elephant seal has a satellite image (on its head) and a time depth recorder (on its back).
Drift dives give the answers
Thanks to a long-term monitoring program led by our colleague Dan Costa, our team has access to diving data from 71 adult elephant seals tagged with small devices, which record time, depth, light, latitude and longitude every four seconds.
It is interesting – and central to this research – when seals perform float diving, fat seals float upwards while lean seals sink. This means that we can use the drift speed of our diving data to calculate the seals’ percentage of body fat over time. Using data on light, depth and time, we can also approach the risk level. In other words, we know whether seals are fat or lean, and we know how much risk it takes. Even better, we both know these statistics continuously throughout the journey.
By measuring body fat at the same time and taking time, we learned that elephant seals take more risks when they were leaner, and put safety first when they were thicker. Early in the food journey, when seals had an average of 22% body fat on average, they rested just after sunrise – 80% of their dives took place during the high-risk day. This enabled them to do most of their feeding at night, when it was easier to get food.
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Later, when the seals filled to 35% body fat, they rested just before sunrise. Only 30% of their dives took place during the high-risk day. Gradual shifts in body condition and behavior during the 220-day food journey increased at the end of the trip to an impressive six-hour shift in the average rest period.
We also discovered that fatter seals rest 300 feet (100 meters) deeper in the water – where it is also 300 times darker – than where thinner seals rested. It further supports the idea that seals strategically change their exposure to light levels – by using rest schedule (time) and rest depth (space) – to minimize the risk. We call it the light image of fear.
Lessons of seals at sea
Our study provides a window into the real-time decision-making of an elephant seal in the open ocean, as it weighs the effects of an afternoon nap below the ocean surface. Although light has previously been identified as a critical constraint on the environment, no study has continuously monitored the use of an animal’s light landscape for extreme shifts in its fat stores and health.
By tracking these criteria together, we were able to better the behavior of a wild animal trying to find food while trying to prevent it from becoming food. Using elephant seals as a model, we can begin to understand how these rules apply to other species – from birds to bats to bears – and scale up to affect entire ecosystems.
This article was published from The Conversation under a Creative Commons license. Read the original article.
Roxanne Beltran receives funding from the National Science Foundation and National Geographic.
Jessica Kendall-Bar receives funding from the National Science Foundation and the Office of Naval Research.
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